Natural killer (NK) cells are a distinctive population of lymphocytes that serve critical functionsin innate immunity, adaptive immunity and reproduction. Key molecular elements in the development, education and effector responses of human NK cells are the interactions between HLA class I molecules and both inhibitory and activating NK cell receptors. The conserved interactions of HLA-E with CD94:NKG2 are complemented by the extraordinarily variable interactions of HLA-A, -B and -C with the killer-cell immunoglobulin-like receptors (KIR), polymorphic ligands and receptors that segregate independently. The combination of HLA class I and KIR genotype uniquely diversifies human immune systems and as a consequence is strongly associated with infectious, allergic, autoimmune and inflammatory diseases as well as with reproductive success and the outcome of clinical transplantation. Many of these associations are with the functionally and genetically distinctive A and B groups of KIR haplotypes that are unique to the human species. Because of the manner by which humans migrated out-of-Africa to colonize the other continents of the world, the genetic diversity of present-day human populations varies over a wide range and involves different subsets of KIR and HLA class I variants. Equally varied is their history of exposure to pathogens. Through the development of new methods for the acquisition of sequence data and its automated analysis, the research proposed in Aim 1 will lead to a complete description of the KIR and HLA class I genes and alleles in human populations worldwide. These data will be analysed to determine how the KIR receptors and HLA ligands have co-evolved in each population and how this differs from one population to another. Aim 2 will investigate a uniquely human KIR gene that emerged early in human evolution, was important in the development of both the KIR A and B haplotypes, but was abruptly inactivated through a single nucleotide insertion. The functionality of this gene will be brought back to life and studied. Aim 3 will develop new high throughput methods that will investigate how the sequences of the peptides bound by HLA class I modulate the interactions with KIR. The initial focus for the application of these methods will be KIR3DL2 recognition of HLA-A*03 and HLA-A*11.